Tires for automobiles in these days are demanded to have various properties such as fuel economy, handling stability, abrasion resistance, and riding comfort. In order to improve those performances, a variety of innovations have been developed. For example, a two-layer structure of a tire tread consisting of a cap tread (surface layer) and a base tread (inner layer) has been known. Diene rubbers such as natural rubber and butadiene rubber are widely used as a rubber component of the base tread. However, when rubber compositions containing natural rubber and/or butadiene rubber are vulcanized with sulfur, a phenomenon of deteriorating the vulcanized rubber composition or of impairing the cross-linking, so-called reversion, may occur.
In these days, vulcanization for tire production has been often performed at a high temperature for a short time so as to improve the productivity of tires. In this case, the aforementioned phenomenon is especially significant. Furthermore, since reversion reduces the modulus and the hardness, the durability and handling stability of tires may be worsened and the tangent δ may be unnecessarily increased, leading to poor fuel economy.
Furthermore, in environmental point of view, like energy saving and tighter regulation on CO2 emission control, an increasing attention has been paid to development of tires with low fuel consumption and there is an urgent need to improve fuel economy. Accordingly, there has also been a demand for a base tread that achieves lower fuel consumption. In order to obtain a tire with low fuel consumption, it is desirable to use a rubber composition with low heat build-up property. Known as a technique for achieving the low heat build-up property is reduction of carbon black in a rubber composition. However, since reduction of carbon black lowers the modulus and the hardness, the aforementioned problems occur.
Patent document 1 discloses a rubber composition containing a mixture of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid, a silica with a certain specific surface area and a silane coupling agent, each in a predetermined amount. The rubber composition can improve rolling resistance performance, processability, abrasion resistance, and wet-skid resistance while suppressing reversion. Patent document 2 discloses a rubber composition which contains a carbon black with predetermined characteristic values and sulfur, and can improve low heat build-up property, chip/chunk resistance, and abrasion resistance in a balanced manner.
However, there is still room for improvement to achieve suppression of reversion, excellent durability and low fuel consumption in a balanced manner. Moreover, application to a base tread is not examined in detail.
Patent document 1: JP-A 2007-321041
Patent document 2: JP-A 2007-131730